1. Field of the Invention
This invention relates to fluoropolymer non-stick coating compositions and to substrates coated with such compositions.
2. Description of Related Art
EP 1 016 466 A2 discloses non-stick coating compositions and their application to substrates to produce adherent, highly abrasion resistant non-stick coatings. In the multi-coat embodiment (fluoropolymer undercoat plus fluoropolymer overcoat), the undercoat contains large ceramic particles which extend from the undercoat to telegraph their presence through the thickness of the overcoat, thereby forming deflection points at the surface of the overcoat. The coating achieves its high abrasion resistance by these deflection points operating to deflect the abrasion force away from the coating. Example 3 discloses the preferred arrangement of the non-stick coating consisting of a primer layer, an intermediate layer and a topcoat, wherein the large ceramic particles are provided by the primer layer. The layer thicknesses are 15-20/15/5-10 micrometers, respectively. The 8.3 wt % total SiC particles in xe2x80x9cwetxe2x80x9d primer composition corresponds to 27.4 wt % SiC in the baked composition. Example 4 repeats Example 3, but replaces the 8.3 wt % (wet basis) SiC particles by SiC particles of different particle size, showing in FIG. 3 that as the particle size increases from 3 to 20 micrometers so does the abrasion resistance. Example 5 shows the effect on abrasion resistance of SiC amount at the same large particle size(s) in the primer composition. The 1 wt %, 3 wt %, 6 wt % and 8.3 wt % amounts (wet basis) of SiC correspond to 4.3 wt %, 12 wt %, 21 wt % and 27 wt % respectively based on the baked weight of the primer composition. As shown in FIG. 4, the coatings in which the primer contained 6 wt % and 8.3 wt % SiC (21 wt % and 27 wt %, baked basis) exhibited much better abrasion resistance than the coatings which contained the lesser amounts of SiC. The primer layer can also contain small particle inorganic film hardener, but preferably, at least 30 wt % of such hardener is the large ceramic particles While the multilayer non-stick coating of EP 1 016 466 excels in abrasion resistance, there is always the desire to have available even better non-stick coatings. The problem confronting the present invention was how to provide a non-stick coating with excellent non-stick property even after being subjected to abrasion, without sacrificing the high abrasion resistance characteristic of the EP 1016 466. In the past, achieving high abrasion resistance for non-stick coatings has meant a sacrifice in non-stick property.
The present invention solves this problem by providing a dynamic non-stick coating, wherein the surface of the non-stick coating rearranges itself in response to abrasion force, rather than being worn away as would be the case if the coating were static. Thus, the exposed surface of the coating is in situ renewable to provide excellent non-stick property. The rearrangement of the non-stick coating is observable with the naked eye by the original smooth unpatterned surface forming a rippled pattern. This rearrangement is further observable in microscopic cross-section by the topcoat layer of the coating becoming mechanically engaged with the midcoat layer of the coating after application of abrasion force to the coating.
Thus present invention can be described as a substrate having a baked non-stick coating thereon, said non-stick coating comprising
(a) a primer layer adhered to said substrate, said primer layer containing fluoropolymer, polymer binder, and inorganic film hardener, said inorganic film hardener including large ceramic particles essentially encapsulated by said primer layer and anchored therein,
(b) a midcoat layer adhered to said primer layer, the encapsulated large particles in said primer layer extending into said midcoat layer, said midcoat layer containing fluoropolymer and particles of inorganic film hardener reinforcing said midcoat layer and entirely contained there within,
(c) a topcoat layer containing fluoropolymer adhered to said midcoat,
the anchoring of said large particles by said primer layer and the reinforcement of said midcoat layer cooperating to translate abrasion force applied to said topcoat layer into mechanical engagement between said topcoat layer and midcoat layer rather than wearing away of said topcoat layer, whereby said topcoat layer remains available in said non-stick coating to provide continuing non-stick property to said coating after application of said abrasion force.
The topcoat layer as originally applied to the midcoat layer in the formation of the non-stick coating forms a smooth but adherent interface between the layers. The mechanical engagement between the topcoat layer and the midcoat layer resulting from the abrasion force includes penetration of the topcoat layer into the midcoat layer. This interlocking relationship helps retain the topcoat layer in the coating. The rearrangement of the topcoat layer is best seen when the coating is subjected to abrasion force applied by the MTP Abrasion Test described later herein, which test includes subjecting the coating to a controlled random reciprocating abrasion. In the region of such abrasion, the smooth unpatterned surface of the topcoat layer is converted to the appearance of a rippled pattern, which is unique for non-stick coatings, including novelty over that of EP 1 016 466 A2. Non-stick coatings heretofore other than that of the European Patent exhibit a poor MTP rating after less than 150 min. of being subjected to the MTP Abrasion Test. Coatings of the present invention (and of the European Patent) exhibit excellent ratings after at least 240 min. in the MTP Abrasion Test, preferably after at least 420 min. As described later herein, the ATP accelerated cooking test reveals the superior abrasion resistance even over the coatings of the European patent.
In one embodiment of the present invention, the non-stick coating relies upon the presence of large ceramic particles being present in the primer layer as does EP 1 016 466, but at less than the optimum amount disclosed in the European Patent, together with a greater reinforcement effect applied by the midcoat layer. Preferably, the primer layer contains 5 to 20 wt % of said large ceramic particles. The primer layer anchors the large ceramic particles within the coating, and this anchoring is reinforced by the midcoat layer. The relatively small amount of large ceramic particles translates to greater spacing between them in their anchored positions. These anchored large ceramic particles resist abrasion force applied to the topcoat, i.e. deflect such force away from the coating. At the same time these anchored particles resist the flow of the topcoat layer in the direction of the applied abrasion force past the anchor points provided by the large ceramic particles.
Preferably the midcoat layer contains at least 8 wt. % of said inorganic film hardener. Despite the greater reinforcement of the midcoat layer of the present invention, as compared to the midcoat layer in the European Patent, which contains 5.4 wt % alumina (Table 8 of EP 1 016 466 A2), based on the baked weight of the layer, the rearrangement of the topcoat layer caused by the abrasion force is translated to movement of the topcoat layer into the midcoat layer, in an interlocking relationship. This together with the anchor protection provided by the large ceramic particles causes the topcoat layer to resist wearing away. Thus, the topcoat layer remains present in the non-stick coating to provide continuing non-stick property, even after being subjected to abrasion.
For simplicity and reflecting the major application of the non-stick coating of the present invention, the non-stick property of the coating will generally be described in terms of food release. Weight percents expressed herein are based on baked weight of the particular layer in question unless otherwise indicated; the baking drives off all volatile materials in the original coating composition, which are therefore not considered in determining such weight percent.
Whereas improved abrasion resistance by a non-stick coating has heretofore been accompanied by sacrifice in food release, the coating of the present invention exhibits both excellent abrasion resistance and excellent food release, even after being subjected to abrasion. Example 3 herein shows the superior food release of the coating of the present invention after abrasion of the topcoat layer.
In another preferred embodiment of the present invention, the midcoat layer is at thicker than said primer layer.
In still another preferred embodiment, the primer layer is relatively thin, e. g. no greater than 14 micrometers thick. Nevertheless the primer layer is able to encapsulate the large ceramic particles and together with the reinforced midcoat layer anchor them firmly in place to resist the abrasion force. The combination of the thin primer layer with the thicker midcoat layer, together with the reinforcement of the midcoat layer transmit the anchoring effect of the large ceramic particles in the primer layer to the midcoat layer, enabling the midcoat layer to, in turn, anchor the topcoat. Preferably, the combination of the midcoat and topcoat layers is at least 200% the thickness of the primer layer. Preferably the total thickness of the coating is no greater than 50 micrometers.
A preferred coating of the present invention can be described as a substrate having a baked non-stick coating thereon, said non-stick coating comprising
(a) a primer layer adhered to said substrate, said primer layer containing fluoropolymer, polymer binder, and inorganic film hardener, said inorganic film hardener comprising large ceramic particles essentially encapsulated by said primer layer and anchored therein and smaller particles totally contained with the thickness of the primer layer, said large particles constituting no greater than 20 wt % of said primer layer.
(b) a midcoat layer adhered to said primer layer, the encapsulated large ceramic particles in said primer layer extending into said midcoat layer, said midcoat layer containing fluoropolymer and at least 8 wt % of particles of inorganic film hardener, said particles being entirely contained within said midcoat layer, said midcoat layer being thicker than said primer layer
(c) a topcoat containing fluoropolymer adhered to said midcoat.
Preferably, said large ceramic particles comprise less than 30 wt % of the total weight of the inorganic film hardener in the primer layer.
Still another embodiment of the present invention is the preferred coating just described which does not require the presence of the film hardener in the midcoat. The combination of the relatively small amount of large ceramic particles in the primer together with the presence of the relatively thick midcoat layer, preferably at least 50% thicker than the primer layer, provides a non-stick coating that exhibits both excellent abrasion resistance and non-stick (food release) character.